1. Field of the Invention
This invention relates to modems, and in particular to training of adaptive structures of a modem.
2. Description of the Relevant Art
Modems are communications devices which employ digital modulation techniques to transmit binary data over analog band-limited communications channels, e.g., telephone lines. Typically, two modems communicate over a single channel, with one modem at each end of the channel. Because the channel exhibits distortion, i.e., non-linear frequency response, equalization is typically employed to compensate for channel impairments, such as amplitude and phase distortion. Additionally, since communication typically occurs in both directions (assuming full-duplex operation), some method of channel separation is typically provided. One method common in modern high speed modems is echo cancellation. In full duplex mode, some of the signal transmitted by a first modem feeds back to the receiver side of the first modem where it appears as a near-end echo signal. In addition, because impedance mismatches in the transmission line are inevitable, some of the first modem's transmitted signal is reflected back from the second modem (and any other points of impedance mismatch) to the first modem as far-end echo. The combination of near- and far-end echo at the first modem's receiver interferes with the signal received from the second modem. An echo canceller operates by creating an approximation of the near- and far-end echo for subtraction from the received signal.
Both equalization and echo cancellation are typically provided by adaptive filter structures which, in traditional modem implementations, are implemented using a custom Digital Signal Processor (DSP) to implement discrete-time filters. Modems incorporating equalization and echo cancellation typically conform to international standards to ensure interoperability with modems from other manufacturers. One such standard is the V.34 specification described in ITU-T Recommendation V.34, A Modem Operating at Data Signalling Rates of up to 28 800 bits/s for Use on the General Switched Telephone Network and on Leased Point-to-Point 2-Wire Telephone-Type Circuits, dated September, 1994 (previously CCITT Recommendation V.34), which is hereby incorporated herein, in its entirety, by reference.
A fractionally spaced equalizer (FSE) usually outperforms a symbol rate equalizer (SRE) due to the FSE's insensitivity to receiver sampling phase. The FSE can be implemented as a passband (or baseband) equalizer following a phase splitter which converts the real received sequence into a complex sequence for input to the FSE. A variant of the phase-splitter/ FSE structure combines the functions of both a phase-splitter and an FSE into one structure, thereby allowing a more efficient implementation which introduces less system delay than the phase-splitter followed by the FSE. This combined structure is known as a phase-splitting FSE (or PS-FSE). See generally, Ling & Qureshi, Convergence and Steady-State Behavior of a Phase-Splitting Fractionally Spaced Equalizer, IEEE TRANS. COMMUN., Vol. 38, No. 4, pp. 418-25 (April 1990). Unfortunately, although the PS-FSE is computationally more efficient than a complex FSE (CFSE), the PS-FSE converges less quickly than an SRE or CFSE.